This invention relates to a new, improved and precise rotary displacement measuring system in which both absolute and relative measurements of rotary position may be made, with compensation for run out (mechanical wobble or misalignment).
In the operation of various mechanical and electromechanical systems, it is necessary to monitor the position and displacement of either some element of the system or some object which is not part of the system. For example, in robotic systems it is almost always necessary to monitor and control the movement and position of various component parts of the systems, such as an arm, fingers or other grasping elements, etc. Such monitoring and control yields the dexterity and precision required for a robotic system to carry out its functions.
Two types of position and displacement measurement may be required in the above-described systems, these being linear displacement and position, and angular or rotary displacement and position. Prior art mechanisms for sensing rotary position and displacement most often utilized a direct connection between the article or object whose position or displacement is to be monitored, and some type of gauge, needle or other visual indicator. Of course, such mechanisms were typically large, cumbersome, unreliable, and lacked precision in carrying out the monitoring function.
A number of electrical and electronic devices have been proposed for measuring angular position and displacement including so-called rotary variable differential transformers, such as described in U.S. Pat. No. 4,910,488. These devices provide for measuring angular displacement by converting mechanical rotation into an analog electrical signal, which may then be connected to a digital output by a conventional A/D converter. In U.S. Pat. No. 4,851,835, a so-called rotary transmitter is disclosed in which rotation of a rotor relative to a pair of stators is determined by variation in capacitance between the rotor and at least one of the stators as the rotor turns. Some prior art rotary measuring devices use several detectors to detect capacitance between the detectors and an emitter but then the outputs from the detectors are integrated to produce a single output signal which is typically analog. As noted above, this analog signal may then be converted to a digital signal for use and interpretation by the user. Finally, prior art devices have typically utilized multiple inputs to excite an emitter and in particular certain sections or segments on the emitter causing it to emit different strength electric fields (usually different phases of an AC excitation signal). Detection of the rotary position of the emitter (or detector) relative to the detector (or emitter) is then made by detecting the variation in strength or phase of the emitted fields.
The above-mentioned electrical and electronic measuring devices, although reducing the bulkiness and imprecision problems of the prior art mechanisms, such devices still do not allow for the precision oftentimes desired in the mechanical and electromechanical systems in use or contemplated for use at the present time.
Other types of rotary displacement and position measuring transducers include resistance-based voltage dividers and optical encoders, both of which also suffer from one or more of the disadvantages mentioned above. For example, while optical encoders have made improvements in the precision of measurements, the devices are still relatively bulky because increased precision is obtained through increasing the number of sensors or other methods which require increased detector surface area.
A further drawback is that the rotary displacement and position measuring transducers above typically use a single type of encoder, either absolute or incremental. The precision of absolute encoders has been poor relative to incremental encoders, but incremental encoders require indexing to find a reference point for movement.
Therefore, it would be an improvement over the prior art to combine the precision of an incremental encoder with the benefits of an absolute encoder so as to provide a rotary displacement and position measuring transducer which is much smaller than the prior art, compensates for run out, and avoids the errors inherent in an analog based system by using geometric quantization to produce a desired digital output representing angular position.